THE GENERAL STRUCTURE OF PLANTS 18 
It is only by its powers of responding to such impressions 
that the whole organism is able to place itself in harmony 
with its environment. Finally, it carries out the processes 
of reproduction. 
The primary needs of a plant are fairly simple. If we 
study the life and the behaviour of one of the free-swim- 
ming organisms of which we have already spoken, we see 
that its first requirement is water. In this it lives; from 
this it draws its supplies of nutriment and into this it pours 
forth its excreta. The arrangement of the protoplasm in 
the cell in one of the higher plants points to a similar need. 
If we regard the arrangement whether in the young or the 
adult cell, we notice particularly the very close relation of 
the protoplasm to water. The young cell enclosed in its cell- 
membrane speedily shows a tendency 
to accumulate water in its interior, 
and gradually drops appear in its sub- 
stance which lead ultimately to the 
formation of a vacuole always full of 
liquid (figs. 15, 16). This store of 
water in the interior of a cell is of 
almost universal occurrence in the 
lowly as well as the highly organised 
plant. The constitution of proto- 
plasm, so far as we know it, depends 
upon this relation, for the appa- 
rently structureless substance is lic. 16.—Apuzr Veorrasie 
always saturated with water. Itis only Sahel Poe Ae 
while in such a condition that a cell 4, cell-wall; , protoplasm: 
can live; with very rare exceptic .s, 24> nuclans with nucleoli; 
if a cell is once completely dried, 
even at a low temperature, its life is gone, and restoration 
of water fails to enable it to recover. 
The constancy of the occurrence of the vacuole in the 
cells of the vegetable organism is itself an evidence that 
such cells are completely dependent upon water for the 
maintenance of life. The cell-wall, though usually 
